Add App.StartScreen to canvas-apps Best Practices (#80)

* Add App.StartScreen guidance to canvas-apps guide and planner agent

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Co-authored-by: lesaltzm <69215460+lesaltzm@users.noreply.github.com>

* Revert App.StartScreen note to Best Practices section only

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Co-authored-by: lesaltzm <69215460+lesaltzm@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lesaltzm <69215460+lesaltzm@users.noreply.github.com>

Author: Copilot

plugins/canvas-apps/references/TechnicalGuide.md

@@ -368,6 +368,9 @@ App:
 
 ## Best Practices
 
+### App Configuration
+- ✅ Set `App.StartScreen` to the initial landing screen so the app always opens on the intended screen
+
 ### State Management
 - ✅ Initialize all variables in `OnVisible`
 - ✅ Use descriptive variable names